Organic semiconducting (OSC) materials are receiving growing interest mostly due to their rapid development in the recent years and the lucrative commercial prospects of organic electronics.
One particular area of importance is that of organic photovoltaics (OPV). Conjugated polymers have found use in OPV as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based OPV devices are reported to achieve a power conversion efficiency (PCE) above 8%.
Benzo[1,2-b:4,5-b′]dithiophene (BDT) polymers have been suggested in prior art for use in OPV devices, and have been reported to show high power conversion efficiency. BDT polymers have been reported for example in U.S. Pat. No. 7,524,922 (Merck), WO 2010/008672 A1 (Univ. Chicago), US 2010/0078074 A1 (Univ. Cal. L.A.), WO 2010/135701 A1 (Polyera), US 2011/0006287 A1 (Univ. N.C.), WO 2011/063534 A1 (Univ. Laval), WO 2011/085004 A2 (Konarka), US 2011/0178255 A1 (Xerox), WO 2011/131280 A1 (Merck), and WO 2011/156478 A2 (Univ. N.C.).
The polymers disclosed in prior art, like for example in the above-mentioned documents, can be categorised into different types of generic backbone structures.
In a first type, the polymer backbone is formed by two electron donating units D (hereinafter also referred to as “electron donor units” or simply “donor units”), like for example BDT, which are separated by an electron accepting unit A (hereinafter also referred to as “electron acceptor unit” or simply “acceptor unit”), and an optional monomer unit M, which is consisting of one or more aromatic units, as shown in formula a) below.*-[(D)a-(A)b]-[(D)c-(M)d]-*  a)
In a second type the polymer backbone is formed by two segments, each consisting of an electron donor unit D, a first spacer unit Sp, an electron acceptor unit A, and a second spacer unit Sp, wherein the spacer units Sp are consisting of one or more aromatic units, such as thiophene, which are not acting as electron acceptors, as shown in formula b) below.*-[(D)a-(Sp)b-(A)c-(Sp)d]x-[(D)a-(Sp)b-(A)c-(Sp)d]y-*  b)
In a third type the polymer backbone is formed by an electron donor unit D, such as BDT, a first spacer unit Sp, an electron acceptor unit A, and a second spacer unit Sp, wherein the spacer units Sp are consisting of one or more aromatic units, such as thiophene, which are not acting as electron acceptors. A polymer of this third type is exemplarily shown in structure 1 below
where R1-4 are substituents like for example alkyl or alkoxy groups.
Thus, in the above-mentioned types of polymers the electron donor unit (like BDT) is not directly connected to an electron acceptor unit in the polymer backbone, but is instead flanked by at least two spacer units. Polymers of these types are disclosed for example in WO 2010/135701 A1, US 2011/0006287 A1, WO 2011/085004 A2, WO 2011/131280 A1 and WO 2011/156478 A2.
In a fourth type the polymer backbone is formed by an electron donor unit, such as BDT, that is directly linked to an electron acceptor unit A. A polymer of this fourth type is exemplarily shown in structure 2 below
wherein R1-4 are as defined above. Thus, in this fourth type of polymers the electron donor unit (BDT) is directly connected to one electron acceptor unit, and the number of donor units in the polymer backbone is at least equal to the number of acceptor units. Polymers of this second type are disclosed for example in WO 2010/008672 A1, US 2010/0078074 A1, WO 2011/063534 A1 and US 2011/0178255 A1.
However, the polymers disclosed in prior art still leave room for further improvements, like a lower bandgap, better processability especially from solution, higher OPV cell efficiency, and higher stability.
Thus there is still a need for organic semiconducting (OSC) polymers which are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, a good processibility, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies, compared to the polymers from prior art.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that are easy to synthesize, especially by methods suitable for mass production, and do especially show good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing conjugated polymers, which are based on a generic co-polymer design that comprises a core structure of a donor unit D, like for example BDT, which is flanked on each side by an electron acceptor unit A, like for example benzothiadiazole, to form a triad A-D-A, and wherein one or more of these triads are optionally separated in the polymer backbone by further spacer units, like for example thiophene.
The inventors of the present invention found that the incorporation of this structural motif into a polymer backbone extends the electron delocalisation, thus leading to a lower band-gap material while maintaining good structural organisation and charge transport properties which are essential for high power conversion efficiency solar cell.
Such a generic co-polymer design concept has not been suggested in prior art so far. JP 2011-124422 A discloses a copolymer of a formula 4 which comprises a 4,8-dihexyl BDT unit and 1-hexyl-1,2-dihydro-pyrazol-3-one units attached to each side of the BDT unit and being separated by 2,2′-bithiophene units, as shown in structure 3 below.

However, the OPV device disclosed in JP 2011-124422 A which comprises this polymer only has a moderate power conversion efficiency of 3.3%. Apart of this specific polymer JP 2011-124422 A does not propose a systematic approach to a generic polymer design where the polymer backbone is composed from “acceptor-BDT-acceptor” triads.
US 2011/0156018 A1 (corresponding to WO 2010/026972 A1) discloses a polymer for the emitting layer of a polymer LED, wherein the polymer backbone is composed of repeating units of structure 4 or 5 below,
wherein Ar1 is a divalent heterocyclic group, X1 and X2 are O, S, NR or CR═CR, X3 and X4 are O, S or NR, and R1-8 are H alkyl, aryl, and the like. In these repeating units two acceptor units, such as benzothiadiazole or benzooxadiazole, are flanking a central unit Ar1, wherein said central unit Ar1 is selected from a broad variety of possible aromatic units, including both donor and acceptor units.
However, US 2011/0156018 A1 does not disclose or suggest polymers that can be used as donor material in OPV cells. Also, it does not describe the generic concept of a polymer containing only triads A-D-A. Besides, it does not contain any specific examples of substituted acceptor units with solubilising groups in R1 to R4 positions are prepared. Also, the examples are limited to thiophene based polymers only.
Furthermore, the above cited prior art documents do not describe any polymer backbones composed of random or statistical block copolymers including A-D-A sequences, segments or blocks.
Also, in prior art hitherto no generic co-polymer design has been disclosed that comprises repeating units containing donor and acceptor units, wherein all donor units are flanked on both sides by an acceptor unit to form a triad A-D-A, and wherein these triads are linked in the polymer backbone by spacer units such as a thiophene.